The invention relates to a method for producing hot-rolled, seamless pipes with thickened ends.
Since the Mannesmann brothers' invention for producing a thick-walled, seamless hollow shell from a heated block, there have been various proposals for stretch-forming this hollow shell, in the same heat in a further hot working step, to a pipe. Keywords in this regard include e.g. the generally known continuous rolling method, push bench method, plug rolling method and pilgering method.
All of the aforementioned methods have their advantages for different dimensional ranges and materials, wherein there are also overlaps. The continuous rolling and plug rolling methods are used for the average dimensional range of 5″ to 18″ and the pilgering method is used for the dimensional range up to 26″.
A characteristic feature of the production of seamless pipes from a heated block by means of hot-rolling are the three steps of piercing—elongation—final rolling with possible subsequent sizing of the diameter of the pipe ends during further processing.
In the case of line pipes which are joined by means of a welded connection to form a continuous run, it is important to be able to weld the pipes together as quickly as possible and without any internal offset of the pipes. One method of achieving this is by mechanical internal machining of the line pipe to an inner diameter of tight tolerance. In this case, in order to ensure that wall thickness does not fall short of the prescribed minimum wall thickness, it is expedient for these pipes to be thickened at the ends prior to mechanical machining on the inner side.
Another example is offered by oil field pipes, in which the individual pipes are joined by means of a thread-connection to form a continuous run. The pipes are provided with an integral thread and are thus screwed together without an additional pipe coupling. The threads introduced into the pipe weaken the pipe, which means that thread-connection points are able to absorb less of a load than the pipe body. Ends which are thickened on the outer and inner circumference render it possible to completely or partially compensate for this deficiency.
It is known that such pipes are thus frequently thickened at their ends in a separate process by means of hot-upsetting.
Such a method is known e.g. from the applicant's patent specification EP 2 170 540 B1 for producing hot-manufactured, seamless pipes, by means of which pipes are produced with optimised fatigue characteristics in the welded state and are welded in an automated manner on a pipe-laying vessel or on land to form pipelines.
In the case of this known method, in a first step a larger wall thickness is produced in one region of the relevant pipe end than in the rest of the pipe body. The wall thickening of the relevant pipe end region is produced by hot-upsetting of the pipe end, wherein the transitions to the pipe body which are produced during upsetting on the outer and inner circumference are arranged in an offset manner in relation to the pipe longitudinal axis.
In a second step, the required pipe cross-section is produced in this region by means of mechanical machining and the transition from the machined region to the un-machined region of the pipe is provided uninterruptedly with such a large radius or with radius combinations that a flowing and notch-free transition is achieved and the finished contour in the originally thickened end region of the pipe has an outer diameter which corresponds to the original diameter of the pipe.
Similar methods, in which thickenings of the pipe ends are produced towards the inside and outside by hot-upsetting and mechanical machining are known e.g. also from laid-open document DE 10 2004 059 091 A1 and patent specification EP 0 756 682 B1.
Further possible ways of producing thickenings by means of rolling technology are known for hot-pilgering. On the one hand, by opening the rolls the roll gap can be enlarged and a thicker wall can be pilgered. Pilger-rolls are sized in such a way that the circular arc cut into the smoothing part has its centre point on the rolling axis and the radius of the circular arc corresponds to the hot diameter of the pipe to be pilgered. The circular arc covers about an extent of 120° per roll. However, when the rolls are being opened a vertical oval is produced (see the explanation hereinafter relating to FIG. 1) which causes the material being rolled to become clamped and produces a more thinly pilgered wall in the flank region compared with the wall in the roll base. For this reason, only thickenings of a few millimetres can be achieved with this method.
Furthermore, from an economic perspective hot-pilgering is not a method which is suitable for the main dimensional range of oil field pipes and line pipes owing to the low quantities of approximately 10 pipes/operating hour of the rolling mill.
For the purpose of producing wall thickenings by means of cold-pilgering, it is proposed in laid-open document DE 31 29 903 A1 to provide the two pilger-rolls with two or even three grooves which are each provided for pilgering the different diameters. Cold-pilgering is an additional processing step which generates considerably more cost than e.g. separate hot-upsetting of the pipe ends and therefore is also not a suitable alternative for producing thickened pipe ends. Moreover, the pipe is only thickened towards the outside and is thus likewise unsuitable for joining line pipes by welding.
For economic reasons, oil field pipes and line pipes are thus mainly rolled at high performance installations, i.e. mandrel bar mills. The objective of mandrel bar mills is to stretch a hot hollow shell, which has been previously produced by skew-rolling, on a rolling mandrel to produce a main pipe. This main pipe is then reduced to the desired final dimension in a sizing or stretch-reducing mill.
Modern mandrel bar rolling mills have in the elongation unit, the actual mandrel bar rolling mill, hydraulic units which use servo-valves to control the settings of the rolls, in order thus to be able to perform positional changes very rapidly. This is currently already being used in order to produce e.g. pipe ends having a slightly reduced wall thickness which are then upset by the process-induced, reduced longitudinal pull during filling and emptying of the rolling stands of the reducing or stretch-reducing mill located downstream, and in this way are then provided at least in part for the pipe material and thus minimise the scrap at the top and bottom.
However, to date it has not yet been possible to produce wall thickenings, in relation to the required nominal wall thickness of the pipe, in a controlled manner over a specific length at the pipe end by means of a mandrel bar mill, as the problems encountered when opening the rolls during pilgering are also present in mandrel bar mills. Moreover, it is desirable, using rolling technology, to produce wall thickenings at specified positions on the pipe e.g. over half the length of the pipe in the case of double lengths, to ensure that, even in the case of double lengths, thickenings are provided at the ends of both pipes.
Furthermore, European patent document EP 1 779 939 B1 already discloses a rolling control method of a multiple-stand mandrel bar rolling mill for pipes. In a conventional manner, the rod rolling mill has a finishing rolling stand. In order to counteract known effects which produce rolled pipes having end regions with a lower wall thickness in comparison with the central regions of the pipe, the rolling control method ensures that the rolls of the finishing stand and of the rolling stand, which as seen in the rolling direction is located upstream thereof and has the same roll-reducing direction as the finishing rolling stand, are opened by a predetermined amount.
Furthermore, German patent DE 11 2013 004 557 T5 discloses a further control method for a pipe mandrel bar rolling mill, in which the length of the rolling stock is measured and/or calculated and in the region of a predetermined spaced interval from the end of the rolling stock the adjustment of the rolling stands is adapted such that the thickness of the pipe along the rolling axis is as constant as possible and also as identical as possible to a desired thickness.